Return of Zombie Ops is a zombie modification for Call of Duty 4, it features content from Black Ops Zombie mode aswell as from Modern Warfare 3 Survival mode and Ghosts Extinction mode. It's set slightly in the future with some weapons from Ghosts, Black Ops series and Modern Warfare series.

Along with Squads there will be 4 Skilltrees, which can be mastered. Each Skilltree will have effects on the player but also on the entire Squad. While Squad members are near players will perform actions faster (rebuild barricades, revive, ...).
You can kinda trade with your squad and also send them points, what should come in handy for buying blockers or just support low level players.
Also some killstreaks will allow multiple players inside, of course you need a squad for that.